It uses the so called Master-Slave Replication approach for synchronizing stored items across all slave shards using Remote Procedure Calls or RPCs, where Master shard has always the most up-to-date version of the container's items. This means that items on all slave shards (Caves for example) are synchronized with Master shard only upon opening the container and only if they are not up-to-date already. So modifying Master's container will not cause synchronization. This saves a lot of unnecessary overhead, especially for servers with multiple shards.

Modifications done to a slave shard container will be sent directly to the Master as soon as they happen. This is because we cannot guarantee that the Master will be able to accept RPCs in the future (e. g. a restart happens).

Items you had in your Shadow Container on a slave shard, orignally before enabling this mod will be dropped on your position before synchronizing it with Master (i. e. before opening it for the first time), so you WON'T loose any them :)